Start up a website and promote affiliate offers.
I run a website geared towards webmasters and people wanting to build websites and I promote hosting services through the affiliate advertisements on the blog.
When someone clicks through my affiliate adverts for the hosting accounts I earn $100.00 per sale if the customer keeps the account and doesn't cancel.
I earn $600.00 per day by just selling 6 of those hosting accounts per day.
Keep in mind though that you won't earn $600.00 per day right away or within the first few years of your blog being online.
It took me 8 years of blogging to earn the $600.00 per day although I did make one or two sales every month after the first year of the blog being online.
You have to work hard building up content on a subject you know a lot about and pick an affiliate program that pays good.
Some bloggers even earn $600.00 per day through Adsense but that requires a boat load of traffic and even more time to build up content on the website.